The present invention relates to a method for performing a frequency analysis of an AC voltage signal, and more particularly an AC voltage signal in a power grid. The present invention also relates to a device for performing the method.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
The AC voltage of power grids has typically a nominal frequency (main frequency). However, the actual frequency frequently deviates from this nominal frequency. For monitoring the AC voltage, devices are used that perform a frequency analysis. In general, these devices use a fast Fourier analysis (fast Fourier transform, FFT). However, in the context of this invention, other transformation methods can also be used, which transform variables in the time domain to variables in the frequency domain, such as the discrete Fourier transformation (DFT) or the Laplace transformation. For a fast Fourier analysis as well as for other transformation methods sampling values must be acquired. To this end, the mains frequency is digitized: a momentarily acting hold circuit (sample-and-hold) freezes the AC voltage signal in periodic intervals, with an analog-digital-converter arranged downstream of the hold circuit for digitizing the input signal. The digitized signal is then supplied to a digital signal processor (DSP) capable of computing the fast Fourier transform and/or scaling and filtering the signal, as well as performing other functions. Current approaches for performing an efficient fast Fourier transform acquire a sequence of sampling values exactly for the duration of a fundamental oscillation of the AC voltage signal. Because the frequency of the AC voltage signal varies, the total sampling duration must also be varied. This is typically preformed concurrently with a readjustment of the frequencies, which requires devices with complex oscillator circuits.
It would therefore be desirable and advantageous to provide a less complex and hence also less expensive device for performing a frequency analysis of an AC voltage signal, such as an AC voltage signal in a power mains, which obviates prior art shortcomings and is able to specifically operate efficiently with fluctuating or changing frequencies of the AC grid.